Control circuitry for an electrical load, such as a motor, generally includes a protection device, such as a fuse, for deactivating the motor based on various operating conditions and the loading on the electric motor. Generally, a protection device is configured to interrupt the current to the motor when the current exceeds a predetermined threshold.
As understood by one of ordinary skill in the art, positive temperature coefficient (PTC) materials are commonly used to protect an electrical circuit from overcurrent conditions. Such materials exhibit an electrical resistivity that is relatively low at a design operating temperature band and increases abruptly, as the temperature of the material rises beyond a critical temperature. As also understood by one of ordinary skill in the art, a PTC overcurrent protection device may be placed in series with a load (e.g., a motor) and under normal operating conditions may operate in a low temperature, low resistance state. However, if the current through the PTC overcurrent protection device increases excessively, the ambient temperature around the PTC overcurrent protection device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, the PTC overcurrent protection device will be “tripped,” and converted to a high temperature, high resistance state such that the current through the PTC overcurrent protection device is substantially reduced.
Referring now to FIG. 1, a simplified block diagram of a conventional motor control system 100 is illustrated. Motor control system 100 includes a power source 102, control circuitry 104, and an electrical load 106, such as a motor. Additionally, control circuitry 104 includes a PTC overcurrent protection device 108. In operation, as the amount of current flowing from power source 102 though PTC overcurrent protection device 108 to electrical load 106 increases, resistive heating of PTC overcurrent protection device 108 occurs and, at a temperature threshold, the resistance of PTC overcurrent protection device 108 increases to limit the amount of current flowing therethrough. When PTC overcurrent protection device 108 cools, it may return to the lower impedance state allowing more current to flow therethrough. Because overcurrent protection utilizing a PTC device involves a thermal process, an unacceptable time delay occurring between application of an unsafe current (i.e., a current above a threshold value) and the limitation of the current may exist. As such, circuit components may be susceptible to unsafe currents and, as a result, may be damaged. Furthermore, once a PTC device is “tripped,” user interaction may be required to reset the device.
A need exists to enhance systems and methods for overcurrent circuit protection. Specifically, there is a need for systems and methods for providing overcurrent circuit protection in a sufficiently fast and variable manner to protect circuit components from dangerous current levels. Additionally, there is a need for systems and methods for automatically restoring a power source connection once an over current condition is no longer present.